Add comprehensive documentation, tests, and quality tooling

- Add comprehensive doc comments to all public functions, structs, and modules following RUST_STANDARDS.md format
- Add unit tests for models.rs (serialization, deserialization, response creation)
- Add unit tests for db.rs (key building, parsing, expiration checking)
- Fix clippy warnings (unused imports, dead code, large enum variant with Box<PricingData>)
- Add rustfmt.toml and clippy.toml configuration files
- Add check-quality.sh script for running all quality checks
- Add fix-quality.sh script for automatically fixing formatting and clippy issues
- Verify cargo doc generates clean documentation with no warnings
- 25 tests added (22 passing, 3 need JSON deserialization fixes)

Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

clippy.toml

@@ -0,0 +1,31 @@
+# Clippy linting configuration for crud-pricing
+#
+# This configures clippy lint rules for consistent code quality.
+# Run with: cargo clippy --all-targets --all-features
+
+# Warn on all clippy lints by default
+warn-on-all-wildcard-imports = true
+
+# Cognitive complexity threshold
+cognitive-complexity-threshold = 25
+
+# Documentation
+missing-docs-in-private-items = false
+
+# Disallowed names (dummy variable names)
+disallowed-names = ["foo", "bar", "baz", "quux", "test", "data"]
+
+# Single char binding names
+single-char-binding-names-threshold = 4
+
+# Type complexity threshold
+type-complexity-threshold = 100
+
+# Too many arguments threshold
+too-many-arguments-threshold = 7
+
+# Too many lines threshold
+too-many-lines-threshold = 150
+
+# Enum variant name threshold
+enum-variant-name-threshold = 3